The following account of the prior art relates to one of the areas of application of the present invention, flexible pipes for marine applications such as the transport of oil or gas or other fluids in a sub-sea environment.
Such a flexible pipe may e.g. comprise an inner liner forming a barrier against outflow of the fluid being transported in the pipe. The inner liner may typically be surrounded by one or more layers of one or more wound armouring profiles. The armouring layers may—depending on the application of the flexible pipe—e.g. be separated by one or more intermediate layers such as membranes. The intermediate layer or layers may be continuous and forming a barrier against outflow of fluids (like the inner liner) or wound and thereby open. The flexible pipe is typically surrounded by an outer layer forming a protective barrier against the outer environment.
In one type of pipes, the armouring layers are not chemically bonded to the inner liner or to possible intermediate membrane layers but can move relative to this or them, which ensures the flexibility of the pipe. This type of pipe is generally termed an ‘un-bonded’ pipe.
The mentioned type of pipe is e.g. used for the transport of oil and gas at large or intermediate sea depths. The mentioned construction is particularly well suited for the transport of oil from sub sea sources to installations at sea level where the oil is being refined or forwarded for further processing.
Often pipes of the above mentioned type comprise an inner liner surrounded by one or more armouring profiles, which are wound around the inner liner at a large angle, e.g. larger than 80°, relative to the center axis of the pipe. This or these layers primarily compensate radial forces in the pipe structure and are here termed ‘radial armouring layers’. The radial armouring layers are surrounded by one or more armouring profiles, which are wound at a considerably smaller angle, e.g. between 25° and 60°, relative to the center axis of the pipe. This or these layers primarily compensate axial forces in the pipe and is/are termed ‘axial armouring layers’. The armouring layers are typically made of steel. A pipe of this kind is in general surrounded by an outer layer forming a protective barrier against the outer environment. Such a pipe is e.g. described in WO 00/36324.
To avoid prohibitively large radial deformations of axial armouring layers due to torsion, axial compression or outer pressures, an outer armouring layer is wound at a large angle around the axial armouring layer(s). This armouring layer is made of very flat profiles, which are here termed ‘tapes’.
To decrease the contact pressure between the armouring layers, a surrounding armouring layer has hence been made of tapes with a precisely balanced strength and stiffness. An outer radial armouring layer made of such tapes fulfils the function of limiting the radial deformations in axial armouring layers of the flexible pipe without considerably increasing the contact pressure on the underlying axial armouring layers. Tapes of this type are e.g. disclosed in US -2004/0025953 and in U.S. Pat. No. 6,165,586.
To ensure that the tapes have a precisely balanced stiffness and sufficient strength to withstand the load on the tape, they are manufactured in several independent processes, where a given reinforcement material, e.g. polyaromatic (aramide) fibres in a certain number are assembled to cords, which subsequently are embedded in a carrier material, e.g. polyethylene, and formed to a tape of a specific width and thickness on a specific manufacturing tool. This solution is very inflexible with regard to constructional and manufacturing freedom for pipes which are constructed and manufactured for specific applications. It thus requires inappropriately many variants of these tapes to be able to cost-effectively manufacture flexible pipes in dimensions from 2 in. to 16 in. (app. 5.1 cm to app. 40.6 cm) in diameter, suited for pressure loads from 50 to 600 bar for use above or below sea level down to 2000 meters in depth.